Support Dask 2.0 (#4617)

python-package/xgboost/dask.py

@@ -91,7 +91,11 @@ def create_worker_dmatrix(*args, **kwargs):
 
 
 def _run_with_rabit(rabit_args, func, *args):
-    os.environ["OMP_NUM_THREADS"] = str(distributed_get_worker().ncores)
+    worker = distributed_get_worker()
+    try:
+        os.environ["OMP_NUM_THREADS"] = str(worker.ncores)
+    except AttributeError:
+        os.environ["OMP_NUM_THREADS"] = str(worker.nthreads)
     try:
         rabit.init(rabit_args)
         result = func(*args)

tests/ci_build/Dockerfile.cpu

@@ -24,7 +24,7 @@ RUN \
     pip install pyyaml cpplint pylint astroid sphinx numpy scipy pandas matplotlib sh recommonmark guzzle_sphinx_theme mock \
                 breathe matplotlib graphviz pytest scikit-learn wheel kubernetes urllib3 && \
     pip install https://h2o-release.s3.amazonaws.com/datatable/stable/datatable-0.7.0/datatable-0.7.0-cp37-cp37m-linux_x86_64.whl && \
-    conda install dask
+    conda install dask=2.0.0
 
 # Install lightweight sudo (not bound to TTY)
 RUN set -ex; \

tests/ci_build/Dockerfile.gpu

@@ -17,7 +17,7 @@ ENV PATH=/opt/python/bin:$PATH
 # Install Python packages
 RUN \
     pip install numpy pytest scipy scikit-learn pandas matplotlib wheel kubernetes urllib3 graphviz && \
-    conda install dask
+    conda install dask=2.0.0
 
 ENV GOSU_VERSION 1.10